3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a wireless device such as a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as an evolved NodeB (eNodeB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. The eNodeB is a logical node in LTE and the RBS is a typical example of a physical implementation of an eNodeB.
FIG. 1 illustrates a radio access network in an LTE system. An eNodeB 101a serves a UE 103 located within the eNodeB's geographical area of service also called a cell 105a. The eNodeB 101a is directly connected to the core network (not illustrated). The eNodeB 101a is also connected via an X2 interface to a neighboring eNodeB 101b serving another cell 105b. Although the eNodeBs of this example network serves one cell each, an eNodeB may serve more than one cell.
The use of a so called heterogeneous deployment or heterogeneous network consisting of radio network nodes transmitting with different transmit power and operating within overlapping coverage areas, is an interesting deployment strategy for cellular networks. In such a deployment schematically illustrated in FIG. 2a, low-power nodes such as pico nodes 210 are typically assumed to offer high data rates measured in Mbit/s, as well as to provide high capacity e.g. measured in users/m2 or in Mbit/s/m2, in the local areas where this is needed or desired. High-power nodes, often referred to as macro nodes 220, are assumed to provide full-area coverage. In practice, the macro nodes 220 may correspond to currently deployed macro cells 221, while the pico nodes 210 are later deployed nodes, extending the capacity and/or achievable data rates in a pico cell 211 within the macro cell 221 coverage area where needed. Pico nodes and macro nodes may also be referred to as pico RBSs and macro RBSs respectively.
In a traditional heterogeneous deployment, schematically illustrated in FIG. 2b, a macro node 220 creates a macro cell 221 and each pico node 210 creates a cell of its own, a so called pico cell 211. This means that, in addition to downlink and uplink data transmission and reception on the pico link 213 maintained between the pico node 210 and the wireless device 212, the pico node 210 also transmits the full set of common signals and channels associated with a cell. In an LTE context this includes the primary and secondary synchronization signals, cell-specific reference signals, and system information (SI) related to the cell, in FIG. 2b referred to as SI pico and illustrated by a cell with a dashed line overlying the pico cell 211.
Alternatively, a terminal or wireless device 212 in the range of a pico node 210, i.e. in the subarea 214 covered by the pico node, may be simultaneously connected to both a macro node 220 and the pico node 210 as illustrated in FIG. 3. To the macro node 220, covering the subarea 222, the terminal 212 maintains a connection or link, e.g. used for radio-resource control (RRC) such as mobility. The connection or link maintained to the macro node 220 may be referred to as an anchor link 223. Furthermore, the terminal 212 maintains a connection or link to the pico node 210, used primarily for data transmission. The connection or link maintained to the pico node 210 may be referred to as a booster link 213.
This approach may be referred to as a combined cell or soft cell approach. In the following it will be referred to as the combined cell approach. The SI related to the combined cell is in FIG. 3 referred to as SI and is illustrated by a cell with a dashed line overlying the subarea 222.
The combined cell approach has several benefits such as mobility robustness and improved energy efficiency. Since the macro layer is responsible for providing e.g. system information and basic mobility management, the pico node in essence only needs to be active when transmitting data to the terminal. This can lead to significant gains in energy efficiency and an overall reduction in interference as the pico nodes can be silent in periods of no data transmission activity. Macro and pico node transmission can either occur on different frequencies in a frequency-separated deployment, or on the same frequency in a same-frequency deployment.
The distinction between cell and transmission and receiving points, often referred to simply transmission points, is an important aspect of the combined cell approach. Each cell has a unique cell identity from which the Cell specific Reference Signal (CRS) is derived. With the cell identity information, a terminal can derive the CRS structure of the cell and obtain the SI it needs to access the network. A transmission point on the other hand is simply one or more collocated antennas from which a terminal can receive data transmissions in a certain area. As a conclusion, a cell may be deployed with one or several antennas or transmission points covering the cell area. In the latter case, the cell is thus served by a plurality of transmission points where each transmission point covers a subarea of the cell.
Configuration of combined cell deployments as well as combinations of such cell deployments with other cell deployments are in LTE done using the following Managed Objects (MO) in a MO model, also illustrated in FIG. 4 (see 3GPP TS 32.792 V10.0.0 (2011-06) and 3GPP TS 32.762 V11.2.0 (2012-06)). A MO may also be referred to as an Information Object Class (IOC):                40: ENodeBFunction—This MO represents eNodeB functionality.        41: EUtranCell—This MO represents the properties of E-UTRAN cell. A cell is a radio network object that can be uniquely identified by a UE from a cell identification that is broadcasted over a geographical area from one UTRAN Access Point. The usage of a sectorEquipmentFunction is defined by attributes in EUtranCell, e.g. the attribute partOfSectorPower. The relation between the ENodeBFunction and the EUtranCell is in FIG. 4 illustrated by the 1 and the star * and the line between the two MOs, which means that one (1) ENodeBFunction may be related to many (*) EUtranCell. The small black rhomb at the end of the line means that the EUtranCell is contained in the ENodeBFunction.        42: SectorEquipmentFunction—This MO represents a set of cells within a geographical area that has common functions relating to AntennaFunction, Tower Mounted Amplifier (TMA) Function, and supporting equipment such as power amplifier. SectorEquipmentFunction thus represents a set of equipment that the set of cells can use. However, the usage of the SectorEquipmentFunction is defined by the EUtranCell. An EUtranCell can only have one SectorEquipmentFunction, as illustrated by the star * and the 1 between the two MOs in the FIG. 4.        44: AntennaFunction—This MO represents an array of radiating elements that may be tilted to adjust the RF coverage of a cell(s).        43: TmaFunction—This MO represents a TMA or a number of TMA subunits within one TMA, each separately addressable by a specific index at the application layer.        
As already described above, FIG. 3 shows a cell configuration with a cell that has several subareas 214, 222, served by two different transmission points 210, 220 using different output power. With the currently existing model, the equipment of both transmission points of this cell would be modeled by one SectorEquipmentFunction and would thus be treated as one single set of equipment in the RBS, as it is only possible for one EUtranCell to have one SectorEquipmentFunction. Furthermore, the usage of the SectorEquipmentFunction is specified by one set of attributes in the EUtranCell A drawback with the current MO model is thus that both antennas/transmission points 210, 220, would have to be configured with e.g. the same output power, the same transmission mode, and the same status. The model is thus not flexible enough to support modeling of combined cell deployments.